The present invention relates to methods of optically extending the dynamic range of sensor arrays for use in optical inspection systems, where surface characteristics and reflectivity variations can cause variations of radiant energy in excess of 60 DB.
In an optical inspection system of the aforementioned species, there is provided a projector module which includes an illumination source and optics required to generate a plane of illumination at the inspection volume. A camera module which is angularly displaced from the projector module includes a TV camera and focusing optics. The focusing optics images that portion of the illumination plane which passes through the inspection volume. The TV camera is positioned so that the plane of its photosensitive array coincides with the camera optics image plane. When an object is placed in the inspection volume so as to intersect the projected plane, a contour of the surface is illuminated and imaged, by the camera optics, on the camera sensitive area. The area array consists of a matrix of photosensitive sites called pixels which are organized into horizontal rows and vertical columns. Electronics circuitry within the camera reads the data in each pixel sequentially across each row until the entire matrix of pixels is read out. The image of the inspection contour on the pixel array results in charge accumulation in those pixels which receive radiant energy from the image. Since the pixels are read out sequentially, the spatial locations of those pixels which were illuminated by the image can be determined. Spatial data on the image plane corresponds via camera optics into spatial data in the object or, in this case, illumination plane. Therefore, from a determination of which pixels were illuminated, spatial information concerning the object contour can be determined. The collection of this spatial data as the object is sequentially moved across the illumination plane results in a set of surface data which completely describes the three-dimensional characteristics of the surface.
To maximize the accuracy of such optical inspections, the pixel illuminated must be accurately determined. Sensors available today have several problems which limit system performance. They include dynamic range limitations of the sensor arrays and charge bleeding or blooming of pixels which are overdriven. The nominal dynamic range presently available from photosensitive arrays is limited to approximately 40 db. However, restraints due to real-time processing further limits the available range resulting in a useful dynamic range of approximately 26 db. Surface characteristics and reflectivity variations can result in energy variations in excess of 60 db. Most objects which have local curvatures or edges can result in reflected energy which varies in excess of 26 db over the inspected height or with angular position with respect to the projected plane. The 26 db sensor capability, therefore, severely limits the inspection capabilities. In addition, when the radiant energy exceeds the dynamic range charge accumulated in the pixel bleeds into adjacent rows and columns degrading or masking the dimensional data. A possible solution to the above problems is to take many inspections at varying transmitted radiant energy levels. This, however, is very time-consuming and requires an object to be stationary during the inspection.